Medications:

Medications which cause dilation of blood vessels cause dark circles under eyes. Dilated blood vessels allow more blood to flow through them and these blood vessels look prominent through under eye skin which is very thin.

Nutrition

Poor nutrition leads to paleness of skin under eyes. Lack of essential nutrients in diet or poor diet causes discoloration of under eye skin.

Hormonal Imbalance

Hormonal Imbalance during pregnancy, menstruation, menopause etc cause paleness of skin and this leads to appearance of dark circles under eyes.